ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Another important function of the lips is to exert pressure on the teeth in the superior
and inferior dental arch. The force balance is obtained by the action of the tongue on
teeth. Tongue muscles push teeth outwards while the lips, when closed, provide
resistance to that force, as their contraction presses teeth intraorally. This force balance
allows dental elements to erupt and remain in the correct position in the oral cavity
(Proffit and Fields, 2002). The orbicularis oris, together with other muscles, act like a
muscle strip, orienting growth of the jaws. When the lips are not sealed, there is no
action of this muscle strip, possibly leading to dysfunctions in jaw growth and teeth
eruption (Gonzalez and Lopes, 2000).
Currently, the evaluation by Speech-language Pathologists of this muscle is routinely
made in a qualitative way. One of the current evaluation techniques consists of asking
the patient to perform a contraction against an imposed obstacle, such as a gloved
finger. Based on their experience, the Speech-language Pathologists classify the force as
normal or not.
Some methods have been developed to quantify the force or pressure that lips are able
to exert, in order to optimize the process of evaluation and rehabilitation in terms of
efficiency and duration (MChenry et al., 1999; Jung et al., 2003; Ruan et al., 2007)
Quantitative objective evaluation can improve diagnosis, especially in cases of slight
changes in force and is more sensitive in detecting small differences in strength
observed with the progression of the disease or therapy.
Recently, image-based models have also been used for the simulation of muscle
contraction (Grasa et al., 2010; Ramírez et al., 2010). The simulation of the lips
contraction will allow knowing more about the movement and displacements of each
part of the lip while the instrument to measure lips force will allow comparing the
values of lip force with the parameters obtained in the qualitative evaluation and to
measure performance during the main functions and exercises.
The purpose of this study is to describe an ongoing research project that includes the
simulation of the lip muscle mechanical behavior and the measurement of lip forces
obtained by a specifically instrument developed for that purpose.
3. METHODS
3.1 Simulation
A three-dimensional geometrical model of the orbicularis oris muscle of a 27 years old
healthy female subject in resting posture was obtained via Magnetic Resonance Imaging
(MRI) and manual segmentation. The woman had no problem or known abnormality in
her orofacial functions and/or structures. On each image, the boundary of the orbicularis
oris muscle was manually outlined, and a three-dimensional polygonal surface model
was generated from the set of two-dimensional outlines using Mimics®.
The mesh (Fig. 1) consisted of 28,740 nodes and 52,474 linear elements, of which
19,082 were hexahedra, 2,641 triangular prisms and 30,751 tetrahedra and it was
generated from the surface model using the Harpoon® Finite Element mesh generator.